Special problems are presented in the manufacturing process of molded articles, which are intended to have internal bores, cavities, or voids. The bores, cavities, or voids of the completed molded article may be filled with other structural components such as springs, connector bushings, and the like. The dimensions of the internal cavities must be established and maintained during the manufacturing process, in order that such inserts may be properly assembled later.
Even more difficult is the manufacturing process where the internal cavities are intended to transmit sound, air, or other fluids where the internal dimensions of the cavities are critical to the properties of the fluids moving therethrough.
The manufacture of molded articles having internal cavities such as stethoscopes accentuate the difficulties of manufacturing because the sound passages must have certain acoustical qualities in order to permit the physician to accurately hear the condition of the patient under examination.
The acoustical passages of the stethoscope, the lumens, may transmit the sound from the patient to one ear of the physician, called a monaural tubing, or to both ears of the physician, called a binaural tubing. The sound passages in the stethoscope may have a common internal cavity, or single lumen, from the patient to the yoke of the stethoscope, whereupon the single lumen divides into two passages, one for each ear of the physician.
Alternately, the stethoscope may have double lumens from the microphone placed against the patient to the yoke of the stethoscope, whereupon the pair of lumens divide, one to each ear of the physician.
As can be appreciated by those skilled in the art, the molding of a long, narrow object, having at least one, and preferably two, internal passageways presents considerable manufacturing difficulties where the acoustical quality of the stethoscope is directly dependent upon the structural configuration of the lumens between the microphone and the ear of the physician. In this regard, U.S. Pat. No. 4,200,169 discusses the importance of the configuration of internal binaural lumen passages and the resolution of acoustical quality in the stethoscope of that invention by the substantial maintenance of a constant diameter throughout the lumens.
Often, stethoscopes are manufactured using the process of hot dip molding. A mold is assembled, heated, and dipped into a liquid plastic or plastisol to permit the plastisol to contact all surfaces of the mold. The amount of plastisol which gels around the mold is related to the heat of the mold when it enters the plastisol reservoir. The mold is withdrawn from the plastisol reservoir and is heated to cure the plastisol into the plastic article comprising the binaural tubing of the stethoscope. The segments of the mold are removed, leaving the internal passages, or lumens, within the binaural tubing. The other components of the stethoscope are attached to the binaural to complete the manufacture of the stethoscope.
Some portions of the mold may require additional thickness of the plastisol in order to provide greater structural stability of the article after molding during usage by the physician. The yoke of the stethoscope, where the lumens separate and are directed to each ear, is a particularly vulnerable portion of the stethoscope during the stress and strain of usage. And the yoke portion of the mold must counteract the force of gravity during the manufacturing process as the plastisol flows on the mold as the mold moves from one area of manufacture to the next. Therefore, that portion of a dip-molding form is subjected to direct, additional heating to achieve additional gelation of the plastisol about that portion of the form to counteract plastisol flow and reinforce a critical junction of the molded article.
The additional steps of direct heating of critical portions of the form to provide additional wall thickness for the molded article is known as the process of "building". Thus, when stethoscopes are manufactured using hot dip molding, it is necessary in the molding of the binaural tubing to "build" sufficient wall thickness at critical portions of the binaural tubing without compromising the desired acoustical quality of the binaural lumens being formed therein.
Currently, as known to those skilled in the art, whenever sufficient wall thickness must be "built", the mold is placed before an open flame in the specific area of the mold requiring "building". Use of an open flame in a manufacturing process is an inexact method at best, even for "build" formation. Atmospheric or climatic conditions, temperature of the flame, and subjective opinions of the manufacturer establishing the processing conditions for the open flame "building" all result in an unpredictible and potentially non-replicating manufacturing process step for the formation of a stethoscope which demands replicated acoustical qualities. Further, the presence of open flames in manufacturing processes represents a potential dangerous manufacturing condition in the presence of any combustible materials or gases.
Molds intended for complete removal after a hot dip molding process are typically made entirely of a material capable of withstanding the temperatures of pre-heating, including "build" temperatures during pre-heating, and curing. However, unfortunately, such heat resistant materials are also rigid and not nearly as flexible as the article molded from the plastisol.
In the case of stethoscopes, the entire length of the binaural tubing must have sufficient flexibility to facilitate use by the health care practitioner. In other words, the molded stethoscope must be flexible along the entire length of its tubing to permit comfortable placement about the head of the health care practitioner and to flex at various locations in order to reach that portion of the body of the patient requiring examination.
If it were desired to avoid the use of an open flame in order to "build" wall thickness at portions of a stethoscope subject to stress and strain during usage, the only molds available to those skilled in the art are heat resistant materials which are rigid and not nearly as flexible as the article to be molded from the plastisol.
Thus, if the rigid mold were left in the stethoscope, after molding was complete, that rigid mold or mold segment would severely interfere with the flexibility and proper use of the stethoscope. Moreover, where a rigid mold or a mold segment is left in the stethoscope, the yoke junction of the stethoscope where the lumens divide would be subject to undesirable pressure and movement as the yoke is flexed.
Hence, what is needed in the art is a method for molding articles which eliminates any necessity of open flame "building" along those portions of the articles which require flexible yet stable junctions, like the yoke portion of the stethoscope. The art also needs a mold which has a member which is both heat resistant and flexible which optionally may remain embedded in the molded article after molding to augment the stability of the molded article at the critical junction, obviating the need for open flame "building" of additional wall thickness.